oh im sorry to hear this but yes 18 yrs old player should get more point than just 5 with normal intensity..
well i dont know if this will work, but try when ur player have full stamina, maybe he tired when u train him, thats why he get less points (maybe)
can u post the picture when u train him?
if he has 5 stars i think just wait until end of season before u sell him,
bcoz 5 stars player can help the team, or if ur squad full of 6 stars player, yes sell him :P